Synthetic antiferromagnet (saf) coupled free layer for perpendicular magnetic tunnel junction (p-mtj)

ABSTRACT

A magnetic tunnel junction (MTJ) device in a magnetoresistive random access memory (MRAM) and method of making the same are provided to achieve a high tunneling magnetoresistance (TMR), a high perpendicular magnetic anisotropy (PMA), good data retention, and a high level of thermal stability. The MTJ device includes a first free ferromagnetic layer, a synthetic antiferromagnetic (SAF) coupling layer, and a second free ferromagnetic layer, where the first and second free ferromagnetic layers have opposite magnetic moments.

FIELD OF DISCLOSURE

The present application for Patent is a divisional of patent applicationSer. No. 14/321,516 entitled “SYNTHETIC ANTIFERROMAGNET (SAF) COUPLEDFREE LAYER FOR PERPENDICULAR MAGNETIC TUNNEL JUNCTION (P-MTJ)” filedJul. 1, 2014, pending, and assigned to the assignee hereof and herebyexpressly incorporated by reference herein.

FIELD OF DISCLOSURE

Various embodiments described herein relate to magnetoresistive randomaccess memory (MRAM), and more particularly, to magnetic tunnel junction(MTJ) in MRAM.

BACKGROUND

MRAM (Magnetoresistive Random Access Memory) is a non-volatile memorythat may utilize MTJ (Magnetic Tunnel Junction) devices, where the stateof an MTJ device depends on the magnetic (electron-spin) orientation ofits ferromagnetic layers. A STT-MTJ (Spin Torque Transfer MTJ) changesthe spin orientation by using a switching current. To achievehigh-density MRAM with good thermal stability and low switching current,attempts have been made to develop MTJ devices with a high perpendicularmagnetic anisotropy (PMA). In a perpendicular magnetic tunnel junction(p-MTJ) having a free ferromagnetic layer, the orientation of themagnetic field in the free ferromagnetic layer is perpendicular to theinterface between the barrier and ferromagnetic layers. It is desirablefor a p-MTJ device to have a high tunneling magnetoresistance (TMR), ahigh PMA, and good data retention.

SUMMARY

Exemplary embodiments of the invention are directed to a magnetic tunneljunction (MTJ) device and method for making the same, with improvedtunneling magnetoresistance (TMR), perpendicular magnetic anisotropy(PMA), data retention, and thermal stability. Moreover, the magnetic andelectrical properties of the MTJ device according to embodiments of theinvention can be maintained at high process temperatures.

In an embodiment, a magnetoresistive random access memory (MRAM) devicecomprises: a first free ferromagnetic layer having a first magneticmoment; a synthetic antiferromagnetic (SAF) coupling layer disposed onthe first free ferromagnetic layer; and a second free ferromagneticlayer disposed on the SAF coupling layer, the second free ferromagneticlayer having a second magnetic moment opposite to the first magneticmoment of the first free ferromagnetic layer.

In another embodiment, a magnetic tunnel junction (MTJ) devicecomprises: a first free ferromagnetic layer having a first magneticmoment; a synthetic antiferromagnetic (SAF) coupling layer disposed onthe first free ferromagnetic layer; and a second free ferromagneticlayer disposed on the SAF coupling layer, the second free ferromagneticlayer having a second magnetic moment opposite to the first magneticmoment of the first free ferromagnetic layer.

In another embodiment, a method for making a magnetic tunnel junction(MTJ) comprises the steps for: forming a first free ferromagnetic layerhaving a first magnetic moment; forming a synthetic antiferromagnetic(SAF) coupling layer on the first free ferromagnetic layer; and forminga second free ferromagnetic layer on the SAF coupling layer, the secondfree ferromagnetic layer having a second magnetic moment opposite to thefirst magnetic moment of the first free ferromagnetic layer.

In yet another embodiment, a method of making a magnetic tunnel junction(MTJ) comprises the steps of: forming a first free ferromagnetic layerhaving a first magnetic moment; forming a synthetic antiferromagnetic(SAF) coupling layer on the first free ferromagnetic layer, the SAFcoupling layer comprising a material selected from the group consistingof ruthenium (Ru) and chromium (Cr); and forming a second freeferromagnetic layer on the SAF coupling layer, the second freeferromagnetic layer having a second magnetic moment opposite to thefirst magnetic moment of the first free ferromagnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are presented to aid in the description ofembodiments of the invention and are provided solely for illustration ofthe embodiments and not limitations thereof.

FIG. 1 is a sectional view of a magnetic tunnel junction (MTJ) in amagnetoresistive random access memory (MRAM) according to embodiments ofthe present invention.

FIG. 2 is a more detailed sectional view of the barrier layer, thesynthetic antiferromagnetic (SAF) coupled free ferromagnetic layerstructure and the capping layer according to embodiments of the presentinvention.

FIGS. 3A and 3B are diagrams illustrating opposite magnetic moments ofthe first and second free ferromagnetic layers of FIG. 2.

FIG. 4 is a flowchart illustrating a method of making an MTJ deviceaccording to embodiments of the present invention.

FIG. 5 is a more detailed flowchart illustrating a method of making anMTJ device according to embodiments of the present invention.

DETAILED DESCRIPTION

Aspects of the invention are disclosed in the following description andrelated drawings directed to specific embodiments of the invention.Alternate embodiments may be devised without departing from the scope ofthe invention. Additionally, well known elements of the invention willnot be described in detail or will be omitted so as not to obscure therelevant details of the invention.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. Likewise, the term “embodiments ofthe invention” does not require that all embodiments of the inventioninclude the discussed feature, advantage or mode of operation.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of embodiments ofthe invention. As used herein, the singular forms “a,” “an,” and “the,”are intended to include the plural forms as well, unless the contextclearly indicates otherwise. It will be further understood that theterms “comprises,” “comprising,” “includes,” or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements, or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, or groups thereof. Moreover, it is understood thatthe word “or” has the same meaning as the Boolean operator “OR,” thatis, it encompasses the possibilities of “either” and “both” and is notlimited to “exclusive or” (“XOR”), unless expressly stated otherwise.

FIG. 1 is a sectional view of a magnetic tunnel junction (MTJ) device100 in a magnetoresistive random access memory (MRAM) which includes asynthetic antiferromagnetic (SAF) coupled free ferromagnetic layerstructure according to embodiments of the present invention. In FIG. 1,a bottom electrode 102 is provided, and a seed layer 104 is disposed onthe bottom electrode 102 in a conventional manner In an embodiment, abottom SAF layer 106 is formed on the seed layer 104, and an SAF layer108, which may comprise ruthenium (Ru) or chromium (Cr), is formed onthe bottom SAF layer 106. In a further embodiment, a top SAF layer 110is formed on the SAF layer 108. In yet a further embodiment, a referencelayer 112 is formed on the top SAF layer 110.

In an embodiment according to the present invention, a barrier layer 114is formed on the reference layer 112. In a further embodiment, thebarrier layer 114 comprises magnesium oxide (MgO). In yet a furtherembodiment, the barrier layer 114 comprises an MgO layer having asurface orientation of (1 0 0), which will be discussed in furtherdetail below with respect to FIG. 2. Other materials may be implementedin the barrier layer 114 without departing from the scope of the presentinvention.

Moreover, the MTJ device 100 comprises an SAF coupled free ferromagneticlayer structure 116, an embodiment of which includes a multi-layerstructure as shown in FIG. 2, which will be discussed in further detailbelow. Referring to FIG. 1, the SAF coupled free ferromagnetic layerstructure 116 is disposed on the barrier layer 114. In an embodiment, acapping layer 118 is formed on the SAF coupled free ferromagnetic layerstructure 116. In a further embodiment, the capping layer 118 comprisesan MgO layer having a surface orientation of (1 0 0). Alternatively, thecapping layer 118 may comprise aluminum oxide (AlO_(x)). Other materialsmay be also be implemented in the capping layer 118 without departingfrom the scope of the present invention. In an embodiment, a topelectrode 120 is formed on the capping layer 118.

FIG. 2 is a more detailed sectional view of the barrier layer 114, theSAF coupled free ferromagnetic layer structure 116 and the capping layer118 according to embodiments of the present invention. In the embodimentshown in FIG. 2, the barrier layer 114 comprises an MgO layer having asurface orientation of (1 0 0). In an embodiment, the SAF coupled freeferromagnetic layer structure 116 comprises a first free ferromagneticlayer 202, which itself comprises a three-layer structure, an SAFcoupling layer 204 formed on the first free ferromagnetic layer 202, anda second free ferromagnetic layer 206, which itself comprises atwo-layer structure, on the SAF coupling layer 204. In an embodiment,the capping layer 118, which may comprise an MgO layer, oralternatively, an AlO_(x) layer, is formed on the second freeferromagnetic layer 206.

In an embodiment, the first free ferromagnetic layer 202 comprises aniron-rich cobalt-iron-boron (Fe-rich CoFeB) layer 202 a formed on thebarrier layer 114. In a further embodiment, the Fe-rich CoFeB layer 202a has an epitaxial relationship with the barrier layer 114 to providehigh tunneling magnetoresistance (TMR) and high perpendicular magneticanisotropy (PMA). In a further embodiment, the Fe-rich CoFeB layer 202 ais subjected to a high-temperature annealing process to transform theFe-rich CoFeB material from an amorphous structure to a crystallinestructure.

In an embodiment, an intermediate layer 202 b is formed on the Fe-richCoFeB layer 202 a. In a further embodiment, the intermediate layer 202 bcomprises a cobalt-iron-boron-tantalum (CoFeBTa) layer. In anotherembodiment, the intermediate layer 202 b comprises acobalt-iron-boron-hafnium (CoFeBHf) layer. Alternatively, anotherelement may be used as an alternative to tantalum (Ta) or hafnium (Hf)in a cobalt-iron-boron (CoFeB) structure in the intermediate layer 202b. In an embodiment, a thin layer of cobalt (Co) 202 c is formed on theintermediate layer 202 b. In a further embodiment, the Co layer 202 c isnot more than 5 Angstroms in thickness.

In an embodiment, the SAF coupling layer 204, which is formed above thethin Co layer 202 c, comprises ruthenium (Ru). Alternatively, the SAFcoupling layer 204 comprises chromium (Cr). Another element may beimplemented in the SAF coupling layer 204 instead of Ru or Cr above thethin Co layer 202 c within the scope of the present invention. The thinCo layer 202 c helps increase SAF coupling to improve the PMA andprevent diffusion of the SAF coupling layer 204 during post annealing.As shown in FIG. 2, the Fe-rich CoFeB layer 202 a, the intermediatelayer 202 b, which may comprise either CoFeBTa or CoFeBHf, and the thinCo layer 202 c together form the first free ferromagnetic layer 202.

In an embodiment, the second free ferromagnetic layer 206 is formed onthe SAF coupling layer 204, which may comprise Ru, Cr or anothermaterial. The second free ferromagnetic layer 206, which is positionedabove the SAF coupling layer 204 opposite the first free ferromagneticlayer 202, enhances the PMA of the MTJ device. In an embodiment, thesecond free ferromagnetic layer 206 comprises a thin Co layer 206 aformed above the SAF coupling layer 204. In a further embodiment, thethin Co layer 206 a has a thickness of not more than 5 Angstroms. In anembodiment, a thin Fe-rich CoFeB layer 206 b is formed on the thin Colayer 206 a. The thin Co layer 206 a and the thin Fe-rich CoFeB layer206 b together form the second free ferromagnetic layer 206. The thin Colayer 206 a increases SAF coupling, improves PMA, and helps prevent Ruor Cr diffusion from the SAF coupling layer 204 during post annealing.Moreover, the thin Fe-rich CoFeB layer 206 b further enhances the PMA ofthe MTJ device.

In an embodiment, the capping layer 118 is formed on the Fe-rich CoFeBlayer 206 b of the second free ferromagnetic layer 206. In anembodiment, the capping layer 118 may be regarded as an integral part ofthe second free ferromagnetic layer 206. As discussed above, the cappinglayer 118 may comprise MgO having a surface orientation of (1 0 0), oralternatively, AlO_(x). In an embodiment, both the barrier layer 114below the first free ferromagnetic layer 202 and the capping layer 118above the second free ferromagnetic layer 206 comprise MgO having asurface orientation of (1 0 0), which is a surface orientation inreference to a planar interfacing surface 203 between the first freeferromagnetic layer 202 and the SAF coupling layer 204, or a planarinterfacing surface 205 between the SAF coupling layer 204 and thesecond free ferromagnetic layer 206.

FIGS. 3A and 3B provide exemplary illustrations of opposite magneticmoments of the first and second free ferromagnetic layers of FIG. 2.FIG. 3A illustrates the SAF coupled free ferromagnetic layer structure116 as part of an MTJ device, acting as a memory cell in an MRAM, withthe first free ferromagnetic layer 202 having a magnetic moment in adirection indicated by an upward-pointing arrow 302, whereas the secondfree ferromagnetic layer 206 having a magnetic moment in a directionindicated by a downward-pointing arrow 304. The upward-pointing arrow302 and the downward-pointing arrow 304 are perpendicular to the planarinterfacing surfaces 203 and 205, and thus the MTJ device illustrated inFIGS. 1 and 2 and described above is called a perpendicular magnetictunnel junction (p-MTJ) device. In FIG. 3B, the first free ferromagneticlayer 202 has a magnetic moment in a direction indicated by adownward-pointing arrow 306, whereas the second free ferromagnetic layer206 has a magnetic moment in a direction indicated by an upward-pointingarrow 308.

In either FIG. 3A or FIG. 3B, the magnetic moment of the first freeferromagnetic layer 202 is opposite to the magnetic moment of the secondfree ferromagnetic layer 206. The SAF coupling layer 204 couples themagnetic orientations of the first and second free ferromagnetic layers202 and 206 such that their magnetic or electron-spin orientations areopposite to each other, thereby resulting in reduced magnetic offset andreduced interference from stray magnetic fields. In an embodiment, thestate of the MRAM memory cell of FIG. 3A, in which the directions 302and 304 of magnetic moments of the first and second free ferromagneticlayers 202 and 206 point toward each other, may be regarded as storing anumber “0,” whereas the state of the MRAM memory cell of FIG. 3B, inwhich the directions 306 and 308 of magnetic moments of the first andsecond free ferromagnetic layers 202 and 206 point away from each other,may be regarded as storing a number “1.” Alternatively, the state of theMRAM memory cell of FIG. 3A may be regarded as storing “1” whereas thestate of the MRAM memory cell of FIG. 3B may be regarded as storing “0.”

FIG. 4 is a flowchart illustrating a method of making an MTJ deviceaccording to embodiments of the present invention. In FIG. 4, a firstfree ferromagnetic layer having a first magnetic moment is formed instep 402. In an embodiment, the first free ferromagnetic layer 202 isformed on a barrier layer 114, for example, an MgO layer having asurface orientation of (1 0 0), as described above with reference toFIG. 2. In an embodiment, the first free ferromagnetic layer 202comprises an Fe-rich CoFeB layer 202 a, an intermediate layer 202 b,which may comprise CoFeBTa or CoFeBHf, and a Co layer 202 c as describedabove with reference to FIG. 2.

Referring to FIG. 4, a synthetic antiferromagnetic (SAF) coupling layeris formed on the first free ferromagnetic layer in step 404. In anembodiment, the SAF coupling layer comprises Ru, or alternatively, Cr,as described above with reference to FIG. 2. Referring back to FIG. 4, asecond free ferromagnetic layer is formed on the SAF coupling layer, thesecond free ferromagnetic layer having a second magnetic moment oppositeto the first magnetic moment of the first free ferromagnetic layer, instep 406. In an embodiment, the second free ferromagnetic layer 206comprises a Co layer 206 a and an Fe-rich CoFeB layer 206 b, asdescribed above with reference to FIG. 2. In a further embodiment, acapping layer 118, for example, an MgO layer having a surfaceorientation of (1 0 0), or alternatively, an AlO_(x) layer, is formed onthe second free ferromagnetic layer 206, as described above withreference to FIG. 2.

FIG. 5 is a more detailed flowchart illustrating a method of making anMTJ device according to embodiments of the present invention. In FIG. 5,a barrier layer comprising MgO is formed in step 502. An Fe-rich CoFeBlayer is then epitaxially grown on the barrier layer in step 504, andthe Fe-rich CoFeB layer is annealed to form a crystalline Fe-rich CoFeBstructure in step 506. In an embodiment, the Fe-rich CoFeB layer issubjected to a high-temperature annealing process to transform theFe-rich CoFeB material from an amorphous structure to a crystallinestructure.

In an embodiment, an intermediate layer comprising a material selectedfrom the group consisting of CoFeBTa and CoFeBHf is formed on theFe-rich CoFeB layer in step 508. A Co layer is then formed on theintermediate layer in step 510. In an embodiment, a thin layer of cobaltwith a thickness of not more than 5 Angstroms is formed on theintermediate layer, which may be either CoFeBTa or CoFeBHf. The Fe-richCoFeB layer, the intermediate layer and the Co layer made according tosteps 504, 506, 508 and 510 together form a first free ferromagneticlayer, such as the first free ferromagnetic layer 202 described abovewith reference to FIG. 2.

Referring to FIG. 5, a synthetic antiferromagnetic (SAF) coupling layeris formed on the first free ferromagnetic layer in step 512. Asdescribed above, the SAF coupling layer may comprise ruthenium, oralternatively, chromium. A Co layer is then formed on the SAF couplinglayer in step 514. In an embodiment, the Co layer formed on top of theSAF coupling layer may be a thin layer of cobalt with a thickness of notmore than 5 Angstroms. Subsequently, an Fe-rich CoFeB layer is formed onthe Co layer in step 516. The Co layer and the Fe-rich CoFeB layer madeaccording to steps 514 and 516 together form a second free ferromagneticlayer, such as the second free ferromagnetic layer 206 described abovewith reference to FIG. 2. In a further embodiment, a capping layer 118,such as an MgO layer with a surface orientation of (1 0 0), oralternatively, an AlO_(x) layer, may be formed on top of the Fe-richCoFeB layer made according to step 516.

While the foregoing disclosure describes illustrative embodiments of theinvention, it should be noted that various changes and modificationscould be made herein without departing from the scope of the inventionas defined by the appended claims. The functions, steps or actions inthe method and apparatus claims in accordance with the embodiments ofthe invention described herein need not be performed in any particularorder unless explicitly stated otherwise. Furthermore, although elementsof the invention may be described or claimed in the singular, the pluralis contemplated unless limitation to the singular is explicitly stated.

What is claimed is:
 1. A method for making a magnetic tunnel junction(MTJ), the method comprising: forming a first free ferromagnetic layerhaving a first magnetic moment; forming a synthetic antiferromagnetic(SAF) coupling layer on the first free ferromagnetic layer; and forminga second free ferromagnetic layer on the SAF coupling layer, the secondfree ferromagnetic layer having a second magnetic moment opposite to thefirst magnetic moment of the first free ferromagnetic layer.
 2. Themethod of claim 1, wherein forming the first free ferromagnetic layercomprises: epitaxially growing an iron-rich cobalt-iron-boron (Fe-richCoFeB) layer on a barrier layer; annealing the Fe-rich CoFeB layer toform a crystalline Fe-rich CoFeB structure; forming an intermediatelayer on the Fe-rich CoFeB layer, the intermediate layer comprising amaterial selected from the group consisting ofcobalt-iron-boron-tantalum (CoFeBTa) and cobalt-iron-boron-hafnium(CoFeBHf); and forming a cobalt (Co) layer on the intermediate layer. 3.The method of claim 1, wherein forming the second free ferromagneticlayer comprises: forming a cobalt (Co) layer on the SAF coupling layer;and forming an iron-rich cobalt-iron-boron (Fe-rich CoFeB) layer on theCo layer.
 4. A method of making a magnetic tunnel junction (MTJ), themethod comprising: forming a first free ferromagnetic layer having afirst magnetic moment; forming a synthetic antiferromagnetic (SAF)coupling layer on the first free ferromagnetic layer, the SAF couplinglayer comprising a material selected from the group consisting ofruthenium (Ru) and chromium (Cr); and forming a second freeferromagnetic layer on the SAF coupling layer, the second freeferromagnetic layer having a second magnetic moment opposite to thefirst magnetic moment of the first free ferromagnetic layer.
 5. Themethod of claim 4, wherein forming the first free ferromagnetic layercomprises: epitaxially growing an iron-rich cobalt-iron-boron (Fe-richCoFeB) layer on a barrier layer comprising magnesium oxide (MgO);annealing the Fe-rich CoFeB layer to form a crystalline Fe-rich CoFeBstructure; forming an intermediate layer on the Fe-rich CoFeB layer, theintermediate layer comprising a material selected from the groupconsisting of cobalt-iron-boron-tantalum (CoFeBTa) andcobalt-iron-boron-hafnium (CoFeBHf); and forming a cobalt (Co) layer onthe intermediate layer.
 6. The method of claim 4, wherein forming thesecond free ferromagnetic layer comprises: forming a cobalt (Co) layeron the SAF coupling layer; and forming an iron-rich cobalt-iron-boron(Fe-rich CoFeB) on the Co layer.